Whoever was in the cockpits of the two airliners that destroyed the World Trade Center didn’t follow secret procedures meant to alert air-traffic controllers to hijackings, The Post has learned.

And the second plane – the one shown in dramatic TV pictures as it shattered the World Trade Center’s south tower – was moving at nearly 600 mph before it hit, said an air-control source.

The lack of any secret warning from the doomed jetliners shows the hijackers’ sophistication, the source said.

“The people who took over knew what they [the pilots] were supposed to do, and didn’t let them,” said the source.

Aircraft are equipped to transmit secret signals to FAA controllers when a hijacking is in progress.

The main indication New York-area controllers had of the impending disaster was that both the planes’ pilots suddenly shifted course – without alerting anyone to their plans.

“His speed was incredibly fast,” the source said of the second plane. “He was about 510 knots [586 mph] all the way to the ground. It just kept descending, and then the target [the aircraft] disappeared,” said the source.